1. Field of the Invention
The present invention relates to a flat cathode ray tube.
2. Description of the Prior Art
As shown in FIGS. 1 and 2, a flat cathode ray tube 6 of a reflection type (or a transmission type) is known in which a flat glass tube 5 is comprised of a three-element structure including a screen panel 1, a front panel (or a back panel) 2, and a funnel portion 4 having a neck portion 3. The flat glass tube 5 is made such that the screen panel 1 and the front panel (or the back panel) 2 are opposed and combined along one end thereof through a frit glass 10 so as to form a flat space 7 therebetween, the funnel portion 4 is combined along its one end to the other end of these panels through the frit glass 10, and the neck portion 3 is welded to the funnel portion 4 at its one open end with a smaller diameter. An electron gun 8 is disposed within the neck portion 3.
The screen panel 1 is configured to be integrally provided with skirt portions 1a and 1b which are erected therefrom along the front end and the opposite side portions thereof continuously. Like the screen panel 1, the front panel (or the back panel) 2 is also configured with skirt portions 2a and 2b which are erected therefrom along the front end and the side portions thereof continuously. The panels 1 and 2 are combined through the frit glass 10 such that the skirt portions 1a and 1b oppose the skirt portions 2a and 2b, respectively.
In a flat cathode ray tube of the reflection type, a phosphor screen 9 is formed on the inner surface of the screen panel 1 through a reflection film formed by vapor deposition of aluminum, for example. An electron beam emitted from the electron gun 8 scans the phosphor screen 9 in the horizontal and vertical directions, so that the phosphor screen 9 is exited by the beam to emit light and hence an optical image formed by the emitted light is viewed from the front panel 2 side opposite the screen panel 1.
In a flat cathode ray tube of the transmission type, the phosphor screen 9 is formed on the inner surface of the screen panel 1 through a transparent conductive film. An electron beam emitted from the electron gun 8 scans the phosphor screen 9 in the horizontal and vertical directions, so that the phosphor screen is exited by the beam to emit light and hence an optical image formed by the emitted light is viewed from the screen panel 1 side opposite to the back panel 2.
If the flat cathode ray tube 6 is manufactured in a manner such that the electron beam is introduced straightly and impinges on the phosphor screen 9 when the electron beam is not subjected to deflection, the phosphor screen 9 may be burnt by ions and so image quality may be greatly degraded. In order to prevent such burning by the ions, the funnel portion 4 is inclined by a predetermined angle and combined to the screen panel 1 and the front panel (or the back panel) 2 so that the electron beam does not impinge on the phosphor screen 9 when the electron beam is not subjected to deflection.
In the flat cathode ray tube 6, when, during operation of the cathode ray tube, the electron beam emitted from the electron gun 8 collides with residual molecules within the flat glass tube 5, ions are generated. Negative ions thus generated by such collisions are not deflected by deflection coils, and these negative ions are accelerated by voltage charged up on the inner surface of the flat glass tube by an anode voltage (about 7 KV) from the flat cathode ray tube 6 and hence are introduced along a locus 11 shown in FIG. 1 and then impinge on the connection of the front end of the flat glass tube 5, that is, around the frit glass 10.
Since the negative ions are not deflected as described above, the negative ions continue to impinge on the same portion, that is, around the frit glass 10 at the front end of the flat glass tube 5 during the operation of the flat cathode ray tube 6. Consequently, the frit glass 10 is heated due to the kinetic energy of the negative ions.
It is clear from the characteristic curve I representing a viscosity of the frit glass 10 shown in FIG. 3, that the frit glass 10 is softened and melted at temperatures lower than those of the glass for the screen panel 1 and the front panel (or the back panel) 2 as shown by a viscosity characteristic curve II thereof. Therefore, when the negative ions impinge on the frit glass 10 for a long period of time, the frit glass 10 is heated and softened, so that there is a risk that the frit glass 10 can not maintain its original function, that is, combining of the glass panels